The invention relates to a vacuum process installation for the surface treatment of workpieces with an arc evaporator source according to the preamble of claim 1 as well as to a method for operating an arc evaporator source according to the preamble of claim 14.
The operation of arc evaporator sources, also known as spark cathodes, by feeding with electrical pulses has already been known in prior art for a relatively long time. With arc evaporator sources high evaporation rates and consequently high deposition rates can be economically attained in coating. The structuring of such a source can, moreover, be technically relatively easily realized as long as higher requirements are not made of the pulse operation and the pulsing is more or less restricted to the ignition of a DC discharge. These sources operate at currents typically in the range of approximately 100 A and more, and at voltages of a few volts to a few tens of volts, which can be realized with relatively cost-effective DC power supplies. A significant disadvantage with these sources comprises that in the proximity of the cathode spot very rapidly proceeding melting occurs on the target surface, whereby drops are formed, so-called droplets, which are hurled away as splatters and subsequently condense on the workpiece and consequently have an undesirable effect on the layer properties. For example, the layer structure thereby becomes inhomogeneous and the surface roughness becomes inferior. With high requirements made of the layer quality, layers generated thusly can often not be commercially applied. Attempts have therefore already been made to reduce these problems by operating the arc evaporator source in pure pulse operation of the power supply. While it has already been possible to partially raise the ionization with pulse operation, however, depending on the setting of the operating parameters, the formation of splatters was additionally even negatively affected.
The use of reactive gases for the deposition of compounds from a metallic target in a reactive plasma was until now only possible within narrow limits, since the problem of splatter formation in such processes is additionally exacerbated, in particular if non-conducting, thus dielectric, layers are to be generated, such as for example oxides using oxygen as the reactive gas. The re-coating of the target surfaces of the arc evaporator and of the counterelectrodes, such as the anodes and also other parts of the vacuum process installation, with a non-conducting layer leads to entirely unstable conditions and even to the quenching of the arc. In this case the latter would have to be repeatedly newly ignited or it would thereby become entirely impossible to conduct the process.
EP 0 666 335 B1 proposes for the deposition of purely metallic materials with an arc evaporator to superimpose onto the DC current a pulsing current in order to be able to lower hereby the DC base current for the reduction of the splatter formation. Pulse currents up to 5000 A are here necessary, which are to be generated with capacitor discharges at relatively low pulse frequencies in the range of 100 Hz to 50 kHz. This approach is proposed for the prevention of the droplet formation in the non-reactive evaporation of purely metallic targets with an arc evaporator source. A solution for the deposition of non-conducting dielectric layers is not stated in this document.
In the reactive coating by means of an arc evaporator source there is a lack of reactivity and process stability, especially in the production of insulating layers. In contrast to other PVD processes (for example sputtering), insulating layers can only be produced by means of arc evaporation with electrically conducting targets. Working with high frequency, such as is the case during sputtering, has so far failed due to the lacking technique of being able to operate high-power supplies with high frequencies. Working with pulsed power supplies appears to be an option. However, in this case the spark, as stated, must be ignited repeatedly or the pulse frequency must be selected so large that the spark is not extinguished. This appears to function to some degree in applications for special materials, such as graphite.
In the case of oxidized target surfaces, repeated ignition via mechanical contact and by means of DC supplies is not possible. Other types of fast ignition processes are technically complex and limited with respect to their ignition frequency. The actual problems in reactive arc evaporation are the coating with insulating layers on the target and on the anode or the coating chamber. These coatings increase the burn voltage of the spark discharge, lead to increased splatters and sparkovers, an unstable process, which ends in an interruption of the spark discharge. Entailed herein is a covering of the target with the growth of islands which decreases the conducting surface. A highly diluted reactive gas (for example argon/oxygen mixture) can slow the accretion on the target, however, it cannot solve the fundamental problem of process instability. While the proposal according to U.S. Pat. No. 5,103,766 to operate the cathode and the anode optionally with a new ignition each time, contributes to process stability, however, it does lead to increased splatters.
The resolution via a pulsed power supply, as is possible for example in reactive sputtering, cannot be applied in a classic spark evaporation. The reason lies therein that a glow discharge has a “longer life” than a spark when the current entry is interrupted.
In order to circumvent the problem of the coating of the target with an insulating layer, in reactive processes for the production of insulating layers either the reactive gas inlet is locally separated from the target (in that case the reactivity of the process is only ensured if the temperature on the substrate also permits an oxidation reaction) or a separation between splatters and ionized fraction is carried out (so-called filtered arc) and the reactive gas after the filtering is added to the ionized vapor. The previous patent application CH 00518/05 shows essentially an approach to a solution to this problem and the invention introduced in the present patent application represents a further development which claims priority of such application, and such is consequently an integrated component of this application.
In contrast to sputtering, coating by means of cathodic spark is substantially an evaporation process. It is supposed that in the transition between hot cathode spot and its margin, portions are entrained which are not of atomic size. These conglomerates impinge as such onto the substrate and result in coarse layers, and it has not been possible fully to react-through the splatters. Avoidance or fragmentation of these splatters was so far not successful, especially not for reactive coating processes. In these forms on the spark cathode, in, for example, oxygen atmosphere, additionally a thin oxide layer, which tends to increased splatter formation. The cited patent application CH00518/05 provided a first solution which is especially well suited for completely reacted target surfaces and has a markedly reduced splatter formation. Nevertheless, a further reduction of the splatters and their size is desirable.
There is further the wish for additional reduction or scaling capability of the thermal loading of the substrates and the ability to conduct low-temperature processes in the cathodic spark coating.
In WO 03018862 the pulse operation of plasma sources is described as a feasible path for reducing the thermal loading on the substrate. However, the reasons offered there apply to the field of sputter processes. No reference is established to spark evaporation.
With respect to prior art, a summary of the following disadvantages is provided:    1. The reactivity in coating by means of cathode arc evaporation is unsatisfactory.    2. There is no fundamental solution of the problematic of splatters: conglomerates (splatters) are not fully reacted-through—>roughness of the coating surface, constancy of the coating composition and stoichiometry.    3. No stable processes are possible for the deposition of insulating layers.    4. The subsequent ionization of splatters is unsatisfactory.    5. Unsatisfactory possibilities of realizing low-temperature processes.    6. Further reduction of the thermal loading of the substrates is unsatisfactory.